Method and apparatus for forming discrete microcavities in a filament wire

ABSTRACT

A microcavity forming device is provided that includes a microcavity forming means of making microcavities in a surface of a heated filament wire. The microcavity forming means includes an array of teeth for engaging the surface along a length dimension of the filament wire. The device further includes a drawing means of drawing the filament wire along the length dimension. The drawing means draws the filament wire along the length dimension and the array of teeth engages the surface of the filament wire to form the microcavities.

FIELD OF THE INVENTION

[0001] This invention relates to forming microcavities in filaments to improve their radiative efficiency. More particularly, this invention relates to a device and method for forming microcavities in a filament that are suitable for mass manufacturing environments.

BACKGROUND OF THE INVENTION

[0002] The cost of producing and purchasing electricity has escalated to all-time highs worldwide. This is especially true in under-developed countries where electricity supply is limited, as well as in those countries with large populations where the demand for electricity is high. Driven by this demand is an ever-increasing desire to produce lighting sources that are energy efficient and minimize the cost of electric usage.

[0003] Over the past two centuries, scientists and inventors have strived to develop a cost-effective, practical, long-life incandescent light bulb. Developing a long-life, high-temperature filament is a key element in designing a practical incandescent light bulb.

[0004] Tungsten filaments have been found to offer many favorable properties for lighting applications, such as a high melting point (3,410° C./6,170° F.), a low evaporation rate at high temperatures (10-4 torr at 2,757° C./4,995° F.), and a tensile strength greater than steel. These properties allow the filament to be heated to higher temperatures to provide brighter light with favorable longevity, making tungsten a preferred material for filaments in commercially available incandescent light bulbs.

[0005] An incandescent lamp filament emits visible and non-visible radiation when an electric current of sufficient magnitude is passed through it. The filament emits, however, a relatively small portion of its energy, typically 6 to 10 percent, in the form of visible light. Most of the remainder of the emitted energy is in the infrared region of the light spectrum and is lost in the form of heat. As a consequence, radiative efficiency of a typical tungsten filament, measured by the ratio of power emitted at visible wavelengths to the total radiated power over all wavelengths, is relatively low, on the order of 6 percent or less.

[0006] Conventional techniques for increasing visible light rely on increasing the amount of energy available from the filament by increasing the applied electrical current. Increasing the current, however, wastes even larger amounts of energy. What is needed is a tungsten filament that emits increased visible light, without increasing energy consumption.

[0007] Another concern is the life span of a filament. A tungsten filament is very durable, but after a prolonged period of time large electrical currents cause excessive electron wind, which occurs when electrons bombard and move atoms within the filament. Over time, this effect causes the filament to wear thin and eventually break.

[0008] It has been observed that the radiative efficiency of filament material such as tungsten may be increased by texturing the filament surface with submicron sized features. A method of forming submicron features on the surface of a tungsten sample using a non-selective reactive ion etching technique is disclosed by H. G. Craighead, R. E. Howard, and D. M. Tennant in “Selectively Emissive Refractory Metal Surfaces,” 38 Applied Physics Letters 74 (1981). Craighead et al. disclose that improved radiative efficiency results from an increase in the emissivity of visible light from the tungsten. Emissivity is the ratio of radiant flux, at a given wavelength, from the surface of a substance (such as tungsten) to radiant flux emitted under the same conditions by a black body. The black body assumes to absorb radiation incident upon it.

[0009] Craighead et al. disclose that the emissivity of visible light from a textured tungsten surface is twice that of a non-textured surface, and suggest that the increase is a result of more effective coupling of electromagnetic radiation from the textured tungsten surface into free space. The textured surface of the tungsten sample disclosed by Craighead et al. has depressions in the surface separated by columnar structures projecting above the filament surface by approximately 0.3 microns.

[0010] Another method for enhancing incandescent lamp efficiency by modifying the surface of a tungsten lamp filament appears in a paper entitled “Where Will the Next Generation of Lamps Come From?”, by John F. Waymouth, pages 22-25 and FIG. 20, presented at the Fifth International Symposium on the Science and Technology of all Light Sources, York, England, on Sep. 10-14, 1989. Waymouth hypothesizes that filament surface perforations measuring 0.35 microns across, 7 microns deep, and separated by walls 0.15 microns thick, may act as waveguides to couple radiation in the visible wavelengths between the tungsten and free space, but inhibit emission of non-visible wavelengths. Waymouth discloses that the perforations on the filament may be formed by semiconductor lithographic techniques, but such perforation dimensions are beyond current state-of-the-art capabilities.

[0011] Another method for reducing infrared emissions of an incandescent light source is described in U.S. Pat. No. 5,955,839 issued to Jaffe et al. As described, the presence of microcavities in a filament provides greater control of directivity of emissions and increases emission efficiency in a given bandwidth. Such a light source, for example, may have microcavities between 1 micron and 10 microns in diameter. While features having these dimensions may be formed in some materials using microelectronic processing techniques, it is difficult to form them in metals, such as tungsten, commonly used for incandescent filaments.

[0012] Yet another method for reducing infrared emissions of an incandescent light source is disclosed in U.S. Pat. No. 6,433,303 issued to Liu et al. entitled Method and Apparatus Using Laser Pulses to Make an Array of Microcavity Holes. The method disclosed uses a laser beam to form individual microcavities in a metal film. An optical mask divides the laser beam into multiple beams and a lens system focuses the multiple beams onto the metal film and forms the microcavities.

[0013] Still another method is disclosed in U.S. Pat. No. 5,389,853 issued, to Bigio et al., and describes a filament having improved emission of visible light. The emissivity of the tungsten filament is improved by depositing a layer of submicron-to-micron crystallites on its surface. The crystallites are formed from tungsten, or a tungsten alloy of up to 1 percent thorium and up to 10 percent of at least one of rhenium, tantalum, and niobium.

[0014] While these conventional methods form microcavities and improve light emissivity, they are complex and costly. None of these methods is suitable for mass manufacturing environments where cost and efficiency are important factors. Consequently, a need still exists for a method of making microcavities in a filament that is suitable for mass manufacturing environments.

SUMMARY OF THE INVENTION

[0015] A microcavity forming device is provided that includes a microcavity forming means of making microcavities in a surface of a heated filament wire. The microcavity forming means includes an array of teeth for engaging the surface along a length dimension of the filament wire. The device further includes a drawing means of drawing the filament wire along the length dimension. The drawing means draws the filament wire along the length dimension and the array of teeth engages the surface of the filament wire to form the microcavities.

BRIEF DESCRIPTION OF THE DRAWING

[0016] The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:

[0017]FIG. 1 is a block diagram of a system for making microcavities in a tungsten filament in accordance with an embodiment of the invention;

[0018]FIG. 2 is a perspective view of a microcavity forming device which forms a portion of the system of FIG. 1, illustrating a pair of rollers in accordance with an embodiment of the invention;

[0019]FIG. 3 is a cross-sectioned view of concave portions of the rollers illustrated in FIG. 2, including teeth for making microcavities in a tungsten filament, in accordance with an embodiment of the invention;

[0020]FIG. 4 is a cross-sectional view of concave portions of a first pair of rollers, similar to the pair illustrated in FIG. 2, and concave portions of a second pair of rollers oriented transversely to the first pair, including teeth for making microcavities in a tungsten filament, in accordance with another embodiment of the invention;

[0021]FIG. 5 is a perspective view of still another embodiment of a microcavity forming device, including multiple pairs of opposing rollers in accordance with the present invention;

[0022]FIG. 6 is an end view of the opposing rollers illustrated in FIG. 5;

[0023]FIG. 7 is a perspective view of yet another embodiment of a microcavity forming device, including a pair of endless tracks having projecting teeth for engaging multiple tungsten filament wires to form microcavities in accordance with the present invention;

[0024]FIG. 7A is an end view of another embodiment of a microcavity forming device, including two pairs of endless tracks positioned downstream from and perpendicularly to each other, both pairs of endless tracks having projecting teeth for engaging a tungsten filament wire to form microcavities in accordance with the present invention;

[0025]FIG. 8 is perspective view of still another embodiment of a microcavity forming device, including multiple pairs of opposing presses having teeth for engaging a tungsten filament wire to form microcavities in accordance with the present invention; and

[0026]FIG. 8A is an end view of another embodiment of a microcavity forming device, including three pairs of opposing presses positioned to move along distinct transverse axes, the opposing presses having teeth for engaging a tungsten filament wire to form microcavities in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Preferred features of embodiments of this invention will now be described with reference to the figures. It will be appreciated that the invention is not limited to the embodiments selected for illustration. Also, it should be noted that the drawings are not rendered to any particular scale or proportion. It is contemplated that any of the configurations and materials described hereafter may be modified within the scope of this invention.

[0028] Referring to FIG. 1, tungsten filament manufacturing system 10 includes heater 14, swaging device 18, microcavity forming device 22, pulling device 26, cutter 28, and coiling device 32. In operation, tungsten material 12 is heated by heater 14 to form heated tungsten material 16. The tungsten is heated by heater 14 to a malleable temperature (1,200° C. to 1,500° C.). The resulting tungsten material 16 is drawn, utilizing swaging device 18, to reduce the diameter of the tungsten material. The heating and drawing steps are repeated until heated tungsten wire 20 of requisite diameter, typically between 2.54 mm and 6.35 mm, is formed. Microcavity forming device 22 is adapted to form microcavities 48 on the outer surface of heated tungsten wire 20. Synchronizer 27 synchronizes the movement of microcavity forming device 22 with the movement of pulling device 26. Cutter 28 cuts microcavitied wire 24 to a final desired length, thereby forming microcavitied filament 30. Finally, microcavitied filament 30 is coiled by coiling device 32 to form filament coil 34. The present invention includes several embodiments of microcavity forming device 22, and is discussed in detail below.

[0029] As will be appreciated, tungsten filament manufacturing system 10 may operate without the use of pulling device 26. Microcavity forming device 22 may duplicate the function of pulling device 26, as explained subsequently with respect to the embodiment illustrated in FIG. 7.

[0030] Referring next to FIG. 2, an embodiment of microcavity forming device 22 is illustrated. Microcavity forming device 22 includes a pair of concave rollers 38, 40. Each roller 38, 40 is rotatably positioned on longitudinal axis A and B, respectively, opposite the other roller. Concave portions 46 are formed circumferentially on external surfaces 50 of each roller 38, 40. Rollers 38 and 40 are positioned such that concave portion 46 of one roller is aligned with concave portion 46 of the other roller thereby forming transverse slot 52 (illustrated in FIG. 3). An array of teeth 54 protrude from each of concave portions 46 of rollers 38 and 40.

[0031]FIG. 3 is a cross-sectional view of concave portions 46 of rollers 38 and 40. Teeth 54, as shown, extend in a radial direction from each of the concave portions. Teeth 54 engage heated tungsten filament wire 20 to form microcavities therein as it is drawn through the transverse slot. The height (x) of teeth 54 is preferably 0.5-0.75 micron, and most preferably 0.5 micron. The width (y) of teeth 54 is preferably 0.5-0.75 micron, and most preferably 0.5 micron.

[0032] In operation, heated tungsten wire 20 exits swaging device 18 and is drawn by pulling device 26 through concave rollers 38 and 40 of microcavity forming device 22. Rollers 38 and 40 rotate on their respective longitudinal axes A and B. Heated filament wire 20 is drawn through transverse slot 52 between rollers 38 and 40. Due to increased malleability from the heating process, as teeth 54 engage the surface of heated tungsten wire 20, microcavities 48 are formed in its outer surface.

[0033] As illustrated in FIG. 2, microcavity forming device 22 may concurrently process from one to five heated tungsten wires 20. It will be appreciated that more wires may be concurrently processed by increasing the number of transverse slots in the pair of rollers.

[0034]FIG. 2 also illustrates an array of teeth 54 as including four rows of teeth protruding circumferentially from the concave portions of the pair of rollers. The present invention, however, may have another number of rows of teeth. Furthermore, the spacing between each tooth in the longitudinal and transverse dimensions of each array may range from 1 to 2 times the width of the tooth.

[0035] As will be appreciated, the vertical spacing between rollers 38 and 40 may be adjusted by moving axis A relative to axis B. The vertical spacing may be adjusted depending on the diameter size of filament wire 20 and the desired depth of microcavities 48. The depth of microcavities 48 are approximately 0.6 times the height (x) of teeth 54. Similarly, the cross-sectional width of microcavities 48 are approximately 1.5 times the width (y) of teeth 54.

[0036] Rollers 38 and 40 may be made from silicon carbide or any other hardened material capable of withstanding the temperature of heated tungsten wire 20, and may be fabricated by micro-machining using a picosecond or femtosecond laser. Similarly, teeth 54 may be made from silicon carbide or any other hardened material, and may be fabricated by micro-machining.

[0037] The microcavity forming device of FIG. 2 may be modified by adding an additional pair of concave rollers oriented transversely and downstream from concave rollers 38 and 40. The additional pair may include opposing concave portions similar to concave portions 46. These two pairs of rollers are illustrated in FIG. 4, which depicts a cross-sectional view of first and second pairs of rollers 56, 58, 60 and 62. First pair or rollers 56 and 58 is rotatably positioned on a respective longitudinal axis C and D, with each respective longitudinal axis opposite the other longitudinal axis. A second pair of rollers 60 and 62, positioned downstream, is rotatably positioned on a respective transverse axis E and F, with each respective transverse axis opposite the other transverse axis. As used herein, the term downstream means further down the production or processing line. Concave portion 64 is formed circumferentially on external surface 66 of each roller 56, 58, 60 and 64. Concave portion 64 of the first pair of rollers 56 and 58 is aligned substantially 90 degrees with respect to concave portion 64 of the second pair of rollers 60 and 62 to form cross-sectional slots 68. An array of teeth 54 protrudes from each of the concave portions.

[0038] Teeth 54, as shown, extend in a radial direction from each of concave portions 46 of rollers 56, 58, 60, and 62. Teeth 54 (which may be the same as teeth 54 of FIG. 3) form microcavities 48 in the heated tungsten filament wire as it passes through microcavity forming device 22. In operation, heated tungsten wire 20 exits swaging device 18 and is drawn by pulling device 26 through concave rollers 56, 58, 60, and 62 of microcavity forming device 22. Rollers 56, 58, 60, and 62 rotate on their respective longitudinal axes C, D, E, and F. Heated filament wire 20 is drawn through cross-sectional slots 68 between rollers 56, 58, 60, and 62. Due to the increased malleability from the heating process, as teeth 54 engage the surface of heated tungsten wire 20, microcavities 48 are formed in its outer surface.

[0039] As will be appreciated, the spacing between rollers 56 and 58 may be adjusted by moving axis C with respect to axis D. Similarly, the spacing between rollers 60 and 62 may be adjusted by moving axis E with respect to axis F. Such spacing between rollers 56, 58, 60, and 62 may be adjusted depending on the diameter size of the filament wire and the desired depth of the microcavities.

[0040] Rollers 56, 58, 60, and 62 may be made from silicon carbide or any other hardened material capable of withstanding the temperature of heated tungsten wire 20. Similarly, teeth 54 may be made from silicon carbide or any other hardened material.

[0041] Referring next to FIG. 5, a further embodiment of microcavity forming device 22 is illustrated. As shown, microcavity forming device 22 includes a first pair of opposing rollers 70 mounted for rotation along a first longitudinal axis G and positioned to rotatably receive heated wire 20 moving in a transverse downstream direction H. A second pair of opposing rollers 72 is mounted for rotation along a second longitudinal axis I and positioned in downstream direction H from the first pair of rollers 70, to rotatably receive the moving heated wire 20. Second longitudinal axis I is angularly displaced from first longitudinal axis G. A third pair of opposing rollers 74 is mounted for rotation along a third longitudinal axis J and positioned in downstream direction H from the second pair of rollers 72 to rotatably receive the moving heated wire 20. Third longitudinal axis J is angularly displaced from second longitudinal axis I. Each opposing roller includes an array of teeth 54 protruding circumferentially from external surface 76 of each roller of each pair 70, 72, and 74.

[0042] Teeth 54, as shown, extend in a radial direction from each of external surfaces 76 of the three pairs of opposing rollers 70, 72, and 74. Teeth 54 form microcavities 48 in a heated tungsten filament wire 20 as it passes through microcavity forming device 22. The height (x) of teeth 54 is preferably 0.5-0.75 micron, and most preferably 0.5 micron. The width (y) of teeth 54 is preferably 0.5-0.75 micron, and most preferably 0.5 micron.

[0043] In operation, heated wire 20 exits swaging device 18 and is drawn by pulling device 26 through three pairs of opposing rollers 70, 72, and 74 of microcavity forming device 22. Opposing rollers 70, 72, and 74 rotate along their respective longitudinal axes G, I, and J. Heated filament wire 20 is drawn through space 78 between the pairs of opposing rollers 70, 72, and 74 in downstream direction H. Due to the increased malleability caused by the heating process, as teeth 54 make contact with the surface of heated tungsten wire 20, microcavities 48 are formed in its outer surface.

[0044]FIG. 6 illustrates an end view of the three pairs of opposing rollers 70, 72, and 74 represented in FIG. 5. The opposing rollers of the first pair 70 are positioned parallel to one another and perpendicular to the path of travel of heated wire 20. The opposing rollers of the second pair 72 are positioned parallel to one another and perpendicular to the path of travel of heated wire 20. Furthermore, second longitudinal axis I of the second pair of opposing rollers 72 is oriented such that it is rotated approximately 315°/135° with respect to first longitudinal axis G of the first pair of opposing rollers 70. The opposing rollers of the third pair 74 are positioned parallel to one another and perpendicular to the path of travel of heated wire 20. Third longitudinal axis J of the third pair of opposing rollers 74 is oriented such that it is rotated approximately 45°/225° with respect to first longitudinal axis G of the first pair of opposing rollers 70.

[0045] As illustrated in FIG. 5, microcavity forming device 22 processes a single element of heated tungsten wire 20 at a given time. It will be appreciated, however, that microcavity forming device 22 may be modified to process more than one tungsten wire. Microcavity forming device 22 may also be modified to include more than three pairs of opposing rollers, each pair angularly displaced from the other pairs. In this manner, more even distribution of microcavities may be formed on a tungsten wire.

[0046]FIGS. 5 and 6 also illustrate an array of teeth 54 as including two rows of teeth protruding circumferentially from the external surface of the opposing rollers of each pair. The present invention, however, may have more than two rows of teeth. The spacing between each tooth in the longitudinal and transverse dimensions of each array may range from 1 to 2 times the width of the tooth.

[0047] As will be appreciated, space 78 between the opposing rollers of each pair 70, 72, and 74 may be adjusted depending on the diameter size of the filament wire and the desired depth of the microcavities.

[0048] The opposing rollers of each pair 70, 72, and 74 may be made from silicon carbide or any other hardened material capable of withstanding the temperature of heated tungsten wire 20. Similarly, teeth 54 may be made from silicon carbide or any other hardened material.

[0049] Referring next to FIG. 7, another embodiment of microcavity forming device 22 is illustrated. Microcavity forming device 22 includes first endless track 80 extending around a first pair of rollers 82 and 84, and positioned for movement parallel to longitudinal dimension K and counterclockwise around roller 82, 84. Rollers 82 and 84 are mechanically and electrically connected to a drive device (not shown) for moving first endless track 80. Second endless track 86 extends around a second pair of rollers 88 and 90. Second endless track 86 is similar to first endless track 80, and is adapted to move parallel to longitudinal dimension K and clockwise around second pair of rollers 88 and 90. Rollers 88 and 90 of the second pair are also mechanically and electrically connected to a drive device (not shown), for moving second endless track 86. First endless track 80 and second endless track 86 are positioned to buttress a plurality of heated wires 20 in space 92. A plurality of teeth 54 protrude from first endless track 80 and second endless track 86.

[0050] Teeth 54, as shown, extend perpendicular from endless tracks 80 and 86. Teeth 54 form microcavities 48 in a heated tungsten filament wire 20 as it passes through microcavity forming device 22. The height (x) of teeth 54 is preferably 0.5-0.75 micron, and most preferably 0.5 micron. The width (y) of teeth 54 is preferably 0.5-0.75 micron, and most preferably 0.5 micron.

[0051] In operation, heated tungsten wire 20 exits swaging device 18 and is drawn by pulling device 26 in between endless tracks 80 and 86 of microcavity forming device 22. Rollers 82, 84, 88, and 90 rotate on their respective longitudinal axes, imparting movements to endless tracks 80 and 86 along longitudinal dimension K. Heated filament wire 20 is drawn through space 92 between endless tracks 80 and 86. Due to the increased malleability caused by the heating process, as teeth 54 make contact with the surface of heated tungsten wire 20, microcavities 48 are formed in its outer surface.

[0052] As will be appreciated, the movements of endless tracks 80 and 86 along longitudinal dimension K may duplicate the function of pulling device 26. Accordingly, the embodiment illustrated in FIG. 7 may operate in the same manner as described previously, without the need for pulling device 26. In other words, heated tungsten wire 20 exits swaging device 18 and is drawn between endless tracks 80 and 86 by the movement imparted to endless tracks 80 and 86 along longitudinal dimension K.

[0053] As illustrated in FIG. 7, microcavity forming device 22 may concurrently process from one to three heated tungsten wires 20. It will be appreciated that microcavity forming device 22 may be modified to concurrently process more than three tungsten wires.

[0054]FIG. 7 also illustrates an array of teeth 54 as including three rows of teeth protruding perpendicular from endless tracks 80 and 86, wherein one row of teeth engages one tungsten wire. The present invention, however, may be modified to include several rows of teeth arranged in an array, wherein each array engages one filament wire to correspondingly form several rows of microcavities in that filament wire.

[0055] As will be appreciated, space 92 between endless tracks 80 and 86 may be adjusted depending on the diameter size of the filament wire and the desired depth of the microcavities.

[0056] Endless tracks 80 and 86 may be made from a flexible conveying-type material capable of withstanding the temperature of heated tungsten wire 20, such as XYZ. Teeth 54 may be made from silicon carbide or any other hardened material, and may be fabricated by a micro-machining system using a picosecond or femtosecond laser.

[0057] The microcavity forming device of FIG. 7 may be modified by adding an additional pair of endless tracks oriented transversely and downstream from endless tracks 80 and 86, as illustrated in end view FIG. 7A. Such an arrangement is capable of processing one tungsten wire at a time. Third and fourth endless tracks 87 and 89, each extending around third and fourth pairs of rollers 91, 93, 95, and 97, respectively, impart movement to the third and fourth endless tracks 87 and 89 along longitudinal dimension K of filament wire 20. Third and fourth endless tracks 87 and 89 are positioned to buttress filament wire 20. Furthermore, third and fourth endless tracks 87 and 89 are positioned downstream from first and second endless tracks 80 and 86, and perpendicularly to first and second endless tracks 80 and 86. Another array of teeth 54 protrudes respectively from third and fourth endless tracks 87 and 89. Similar to first and second endless tracks 80 and 86, third and fourth endless tracks 87 and 89 are adapted to move with filament wire 20 along longitudinal dimension K, and the other arrays of teeth engage the moving filament wire 20 to form microcavities 48 in filament wire 20.

[0058] Referring next to FIG. 8, a further embodiment of microcavity forming device 22 is illustrated. As shown, microcavity forming device 22 includes a first pair of opposing presses 92, in which each opposing press includes first contoured plate 94 conforming to a portion of an outer surface of filament wire 20, which is drawn along longitudinal axis N. The first pair of opposing presses 92 is mechanically connected to a reciprocating drive device (not shown) and is adapted to move in a first transverse axis O. A second pair of opposing presses 96 is positioned along longitudinal axis N and downstream from the first pair of opposing presses 92. Each opposing press includes second contoured plate 98 conforming to another portion of the outer surface of filament wire 20. The second pair of opposing presses 96 is also mechanically connected to a reciprocating drive device (not shown) and is adapted to move in a second transverse axis P which is angularly displaced from the first transverse axis O. An array of teeth 54 project outwardly from each first contoured plate 94 and second contoured plate 98.

[0059] Teeth 54, as shown, extend in a radial direction from first contoured plate 94 and second contoured plate 98. Teeth 54 form microcavities 48 in a heated tungsten filament wire 20 as they engage the filament wire. The height (x) of teeth 54 is preferably 0.5-0.75 micron, and most preferably 0.5 micron. The width (y) of teeth 54 is preferably 0.5-0.75 micron, and most preferably 0.5 micron.

[0060] In operation, heated tungsten wire 20 exits swaging device 18 and is drawn by pulling device 26 through the first pair of opposing presses 92 and the second pair of opposing presses 96 of microcavity forming device 22. Pulling device 26 includes start and stop controls for, respectively, starting and stopping the drawing of filament wire 20 along length dimension N. First pair of opposing presses 92 is moved along first transverse axis O towards heated wire 20, and similarly second pair of opposing presses 96 is moved along second transverse axis P towards heated wire 20. Such movement of first and second pairs of opposing presses 92 and 96 is synchronized with the starting and stopping of the drawing of filament wire 20. In sequence, pulling device 26 draws filament wire 20, stops the drawing of filament wire 20, and contoured plates 94 and 98 of opposing presses 92 and 96 engage heated wire 20. Due to the increased malleability caused by the heating process, as teeth 54 make contact with the surface of the heated wire, microcavities 48 are formed in its outer surface. Opposing presses 92 and 96 are then released along their respective transverse axes O and P away from microcavitied tungsten filament material 24.

[0061]FIG. 8 illustrates microcavity forming device 22 as including two pairs of opposing presses 92 and 96. The present invention, however, may be modified to include additional pairs of opposing presses, each pair of opposing presses moving along a distinct transverse axis, so that additional microcavities may be formed about the surface of the filament wire. For example, an additional pair of opposing presses 100 is illustrated in end view FIG. 8A.

[0062] The opposing presses of each pair 92 and 96 may be made from silicon carbide or any other hardened material capable of withstanding repeated stamping pressure and the temperature of heated tungsten wire 20. Similarly, teeth 54 may be made from silicon carbide or any other hardened material.

[0063] The present invention provides an improvement over conventional methods of forming microcavities in a filament, as it is suitable for mass manufacturing environments where cost and efficiency are important factors. The present invention does not require complicated and costly laser devices, and instead utilizes simple mechanical structures to form microcavities. A further benefit of the present invention is that it creates numerous microcavities in a predetermined pattern. The present invention may also be implemented with minimum changes to a conventional filament manufacturing production line.

[0064] It will be appreciated that other modifications may be made to the illustrated embodiments without departing from the scope of the invention, which is separately defined in the appended claims. 

1. A microcavity forming device comprising: microcavity forming means of making microcavities in a surface of a heated filament wire, the microcavity forming means including an array of teeth for engaging the surface along a length dimension of the filament wire, and drawing means of drawing the filament wire along the length dimension, wherein the drawing means draws the filament wire along the length dimension and the array of teeth engages the surface of the filament wire to form the microcavities.
 2. The device of claim 1 wherein the microcavity forming means includes: a pair of rollers, each roller rotatably positioned on a longitudinal axis opposite the other roller, at least one concave portion formed circumferentially on an external surface of each roller of the pair of rollers, the concave portion of one roller of the pair of rollers being aligned with the concave portion of the other roller of the pair of rollers to form a transverse slot, and the array of teeth protruding from each of the concave portions, wherein, as the pair of rollers rotate on respective longitudinal axes, a heated filament wire is drawn through the transverse slot, and the array of teeth engages the filament wire to form the microcavities in the filament wire.
 3. The device recited in claim 2, wherein the array of teeth includes at least two rows of teeth protruding circumferentially from the concave portion.
 4. The device of claim 2, wherein the rollers are made of silicon carbide.
 5. The device of claim 1 wherein each tooth of the array of teeth has a height of 0.2-1.5 microns and a cross-section having a width of 0.2-1.5 microns.
 6. The device of claim 1 wherein each tooth of the array of teeth has a height of 0.5-0.75 micron and a cross-section having a width of 0.5-0.75 micron.
 7. The device of claim 1 wherein each tooth of the array of teeth has a height of 0.5 micron and a cross-section having a width of 0.5 micron.
 8. The device of claim 1 wherein a spacing between each tooth of the array of teeth ranges from 1 to 2 times a width of the tooth.
 9. The device of claim 1 wherein each microcavity formed in the filament wire includes a cross-sectional width of approximately 1.5 times a width of the tooth.
 10. The device of claim 1 wherein each microcavity formed in the filament wire includes a depth of approximately 0.6 times a height of the tooth.
 11. The device recited in claim 1, wherein each tooth in the array is made of silicon carbide.
 12. The device of claim 1 wherein the microcavity forming means includes: a first pair of rollers, each roller of the first pair rotatably positioned on a respective longitudinal axis, each respective longitudinal axis being opposite the other longitudinal axis, a second pair of rollers, each roller of the second pair rotatably positioned on a respective transverse axis, each respective transverse axis being opposite the other transverse axis, the second pair of rollers disposed downstream of the first pair of rollers, a concave portion formed circumferentially on an external surface of each roller of the first and second pairs, the concave portion of a roller of the first pair being aligned at an angle of substantially 90 degrees with respect to the concave portion of a roller of the second pair, the concave portions of each pair of rollers forming a cross sectional slot, the array of teeth protruding from each of the concave portions, wherein, as the first and second pairs of rollers rotate respectively, on the longitudinal axes and transverse axes, a heated filament wire is drawn through each of the slots, and the array of teeth engages the filament wire to form the microcavities in the filament wire.
 13. The device of claim 12, wherein the rollers are made of silicon carbide.
 14. The device of claim 1 wherein the microcavity forming means includes: a first pair of opposing rollers mounted for rotation along a first longitudinal axis and positioned to rotatably receive a heated filament wire moving in a transverse downstream direction, a second pair of opposing rollers mounted for rotation along a second longitudinal axis and positioned downstream from the first pair of rollers to rotatably receive the moving heated filament wire, the second longitudinal axis being angularly displaced from the first longitudinal axis, and each opposing roller including the array of teeth protruding circumferentially from an external surface of each roller, wherein the pairs of rollers rotatably receive the moving filament wire and the array of teeth engages the filament wire to form the microcavities in the filament wire.
 15. The device of claim 14, wherein the rollers are made of silicon carbide.
 16. The device of claim 14 wherein the microcavity forming means includes: a third pair of opposing rollers mounted for rotation along a third longitudinal axis and positioned downstream from the second pair of rollers to rotatably receive the moving heated filament wire, the third longitudinal axis angularly displaced from the second longitudinal axis.
 17. The device of claim 1 wherein the microcavity forming means includes: a first endless track extending around a first pair of rollers, in which the first pair of rollers imparts a movement to the first endless track along a longitudinal dimension and around the first pair of rollers, a second endless track extending around a second pair of rollers, in which the second pair of rollers imparts a movement to the second endless track along the longitudinal dimension and around the second pair of rollers, the first and second endless tracks positioned to buttress the heated filament wires, and the array of teeth protruding from the first and second endless tracks, wherein the first and second endless tracks are adapted to move with the heated filament wire along the longitudinal dimension, and the array of teeth engage the moving filament wire to form the microcavities in the filament wire.
 18. The device of claim 17 wherein the microcavity forming means further includes: third and fourth endless tracks, each extending around third and fourth pairs of rollers, respectively, for imparting movements to the third and fourth endless tracks along the longitudinal dimension of the filament wire, the third and fourth endless tracks positioned to buttress the filament wire, and positioned downstream from the first and second endless tracks, and perpendicularly to the first and second endless tracks, and another array of teeth protruding respectively from the third and fourth endless tracks, wherein the third and fourth endless tracks are adapted to move with the filament wire along the longitudinal dimension, and the other arrays of teeth engages the moving filament wire to form the microcavities in the filament wire.
 19. The device of claim 17 wherein the drawing means includes driving means of rotating the first and second pairs of rollers, and the first and second endless tracks drawing the filament wire as the first and second pairs of rollers are driven by the driving means.
 20. The device of claim 1 wherein the microcavity forming means includes: a first pair of opposing presses, in which each opposing press includes a first contoured plate conforming to a portion of an outer surface of a filament wire, the filament wire having a longitudinal axis, and the first pair of opposing presses adapted to move in a first transverse axis; a second pair of opposing presses, positioned downstream along the longitudinal axis from the first pair of opposing presses, in which each opposing press includes a second contoured plate conforming to another portion of the outer surface of the filament wire, and the second pair of opposing presses adapted to move in a second transverse axis, the second transverse axis angularly displaced from the first transverse axis; and the array of teeth projecting outwardly from each first and second contoured plates; wherein a heated filament wire is moved along the longitudinal axis and the first and second pair of opposing presses are moved, respectively, in the first and second transverse axes, and the array of teeth engage the heated filament wire to form the microcavities.
 21. The device of claim 20 wherein the opposing presses are made of silicon carbide.
 22. The device of claim 20 in which the drawing means includes start and stop controls for, respectively, starting and stopping the drawing of the filament wire along the length dimension, and further including synchronization means of synchronizing the movement of the first and second pairs of opposing presses with the starting and stopping of the drawing of the filament wire, wherein, in sequence, the drawing means draws the filament wire, stops the drawing of the filament wire, and the first and second pairs of opposing presses engage the filament wire.
 23. The device recited in claim 20 wherein the microcavity forming means further includes: a third pair of opposing presses positioned downstream along the longitudinal axis from the second pair of opposing presses, and each opposing press including a third contoured plate conforming to another portion of the outer surface of the filament wire; the third pair of opposing presses adapted to move in a third transverse axis, and the third transverse axis angularly displaced from the second transverse axis; and an array of teeth projecting outwardly from each third contoured plate; wherein a heated filament is moved along the longitudinal axis and the third pair of opposing presses is moved in the third transverse axis, and the array of teeth engages the heated filament wire to form the microcavities.
 24. A method of making microcavities in a filament wire comprising the steps of: (a) positioning each roller of a pair of rollers on a longitudinal axis opposite the other roller, in which each roller includes a concave portion formed circumferentially on an external surface, and further includes an array of teeth projecting from each respective concave portion; (b) aligning the respective concave portions to form a transverse slot; (c) rotating each roller on the respective longitudinal axis; (d) drawing a heated filament wire through the transverse slot; and (e) engaging the array of teeth with the heated filament wire to form the microcavities in the filament wire as each roller rotates in step (c).
 25. A method of making microcavities in a filament wire comprising the steps of: (a) positioning each roller of a first pair of rollers on a respective longitudinal axis opposite each other, in which each roller includes a concave portion formed circumferentially on an external surface, and further includes an array of teeth projecting from each respective concave portion; (b) positioning each roller of a second pair of rollers downstream from the first pair of rollers on a respective transverse axis opposite another roller of the second pair of rollers, in which each roller includes a concave portion formed circumferentially on an external surface, and further includes an array of teeth projecting from each respective concave portion; (c) aligning the concave portion of a roller of the first pair substantially 90 degrees with respect to the concave portion of a roller of the second pair, the concave portions of each pair of rollers forming a cross sectional slot; (d) drawing a heated filament wire through each slot; and (e) engaging the array of teeth with the heated filament wire to form the microcavities in the filament as each roller rotates in response to drawing the heated filament wire through each slot.
 26. A method of making microcavities in a filament wire comprising the steps of: (a) positioning a second pair of opposing rollers downstream from a first pair of opposing rollers in which each roller of the first and second pairs of rollers includes an array of teeth projecting from an external surface of each opposing roller; (b) aligning the first and second pairs of opposing rollers along first and second longitudinal axes, respectively, in which the first and second longitudinal axes are angularly displaced from each other; (c) rotating each roller of the pairs of opposing rollers; (d) drawing a heated filament wire through the pairs of opposing rollers; and (e) engaging the array of teeth with the heated filament wire to form the microcavities in the filament wire, as each roller rotates in step (c).
 27. The method recited in claim 26, further comprising the steps of: (f) positioning a third pair of opposing rollers downstream from the second pair of opposing rollers, in which each roller of the third pair of opposing rollers includes an array of teeth projecting from an external surface of each opposing roller; (g) aligning the third pair of opposing rollers along a third longitudinal axis, in which the third longitudinal axis is angularly displaced from the first and second longitudinal axes.
 28. A method of making microcavities in a filament wire comprising the steps of: (a) positioning first and second endless tracks to buttress a plurality of heated filament wires between the first and second endless tracks in which each of the first and second endless tracks extends around, respectively, first and second pairs of rollers, and includes a plurality of teeth projecting from each surface of the first and second endless tracks; (b) rotating the first and second pairs of rollers to impart movements to the first and second endless tracks along a longitudinal axis; (c) drawing the heated filament wires between the first and second endless tracks; and (d) engaging the plurality of teeth with the heated filament wires to form the microcavities in the filament wires, as the first and second endless tracks are moved.
 29. A method of making microcavities in a filament wire comprising the steps of: (a) positioning a second pair of opposing presses downstream along a longitudinal axis from a first pair of opposing presses in which each opposing press includes a contoured plate having an array of teeth projecting therefrom; (b) moving the first and second pairs of opposing presses in opposition along first and second transverse axes, respectively, the first transverse axis angularly displaced from the second transverse axis; (c) drawing a heated filament wire along the longitudinal axis and between the contoured plates of the pairs of opposing presses; and (d) engaging the arrays of teeth with the heated filament wire to form the microcavities in the heated filament wire.
 30. The method of claim 29 further including the steps of: (e) positioning a third pair of opposing presses along the longitudinal axis and downstream from the second pair of opposing presses in which each opposing press includes a contoured plate having an array of teeth projecting therefrom; and (f) moving the third pair of opposing presses in opposition along a third transverse axis, the third transverse axis angularly displaced from the first and second transverse axes;
 31. A method of making a roller for forming microcavities in a heated filament wire, the method comprising the steps of: (a) forming an external surface of a cylindrical structure with silicon carbide; and (b) projecting circumferentially an array of teeth from the external surface, in which each tooth of the array is made from silicon carbide.
 32. The method of claim 31 in which forming the external surface includes shaping the external surface to include at least one circumferential concave portion, and projecting the array of teeth from the shaped circumferential concave portion. 